A person playing prerecorded music for people to enjoy, for example in clubs and discotheques will be referred herein as a Disc Jockey or DJ.
The following terms regarding music are defined:
Rhythm of music is the organization of an audio track in relation to time.
A beat is the basic unit of duration in an audio track.
A measure is a notation of a longer pattern than a beat within the track. The duration of one measure is usually equal to some fixed number of beats.
The time signature, written similarly to a fraction, sets the rhythmic context, called meter. The numerator tells the duration of a measure, in beats (common are 2,3,4,6) while the denominator tells the length of one beat in the measure.
Tempo describes how fast the rhythm is played, or how the beats correspond to absolute temporal durations. Tempo can be specified by beat frequency, beats per minute, hereafter BPM. Since there are 60 seconds per minute, BPM/60 equals HZ (cycles per second). The BPM of a track is usually constant, but there are occurrences where the BPM may vary within the audio track.
A Downbeat is referred to the first beat of a measure.
Herein an audio track may refer to a song.
Scratching is referred to as a physical manipulation of a recorded sound, which is accomplished by a manual, back and forth playing of that recorded sound. For example, with a record player by holding the played record (disconnecting it from the spinning motor) and then moving the record back and forth against the needle at a specific point in the record, to make a special sound effect.
The specialty of a DJ is in selecting musical tracks to play to an audience and in “mixing” the musical tracks in a manner that may be pleasing to the listeners' ears. “Mixing” two tracks involves playing one track and then introducing the next track in such a manner that the transition to the next track is musically smooth. To the perception of the audience, one long musical stream is being played, not a collection of discrete tracks. Without mixing, a certain silence or jump in tempo may occur after each track.
Mixing is accomplished by playing the end of track A at the same time as track B starts. This overlap can create a new problem of mismatched beats, creating a clutter of two audio tracks, played simultaneously on two separate independent machines. The DJ attempts to make a smooth transition between the played tracks. The DJ matches the next track to the currently playing one in such a way that there may be no audible notice of a change of track. The audible sound is of a mix between the present track and the next one entering.
This matching between two different audio tracks is called mixing or beat mixing.
Normally, when the playing track, also referred to as track A or current track, is about to end, a new track, also known as track B or entering track, has to be introduced. Beat mixing is accomplished by matching the beats of the two audio tracks A and B. This matching is done by manipulating audio tracks by speeding them up or slowing them down, using a special control called a pitch controller, hence changing the audio track tempo of one or both the tracks until both tracks have the same tempo. In addition to matching the tempo, the timing of the beats of both tracks must be simultaneous, so that one track does not lag after the other. This is done by entering the downbeat of the entering track B at the time when a downbeat of the playing track A occurs.
One way of accomplishing beat mixing is by playing records using record turntables with a pitch controller. A pitch controller controls the revolutions per minute of the turntable, which changes the BPM of the played audio track. A DJ playing records can mix between two audio tracks by beat matching two audio tracks manually using the pitch controller on the turntable of the entering track. Mixing is initiated by entering the downbeat of the entering track at the downbeat of the playing track. Since both the new track entry and the beat matching process are manual, they are therefore imperfect and corrections are necessary so that the tracks will match one another exactly. These corrections are made by touching the turntable (clockwise to speed up or lightly touching to slow down) and further adjusting the pitch of the entering audio track, while both tracks are played. This sometimes leads to small audible glitches and other times to a complete out-of-beat mix.
In spite of these limitations, the record format is generally favored among DJ's due to the fact that the music's progress in time can be physically manipulated by the direct touch of a hand. This is done by either touching the record and speeding its spin compared to the motor speed, or slowing it down by touching the motor lightly.
CDs with mixing capabilities especially for DJs called CDJs have been introduced. The CDJ attempts to match the capabilities of turntable mixing, by offering a manual pitch controller and a rotational plate that can manipulate the played track in a way similar to touching the record as it is played.
The Digital Compact Disc (CD) is ROM (Read Only Memory) so that pinpointing inside a track is possible, meaning that there is a direct access to an exact point for each track. The CD media is different from the record media in that it is random access. This means that exact locations within the track can be marked and these locations can be used to skip between locations.
One way of using the random access capability of the CD media is know as a loop. A loop in a track is a repetition of a partial part of a track (track segment) initiating from one point A in the track and ends at another point B in the track. The loop must be made in such a way so that when a repetition is made and the track jumps back from point B to point A, the sound is smooth and there is no noticeable “glitch” to the audience. The choosing of points A and B is therefore done by marking a full number of measures, starting at the downbeat of a measure and ending just before the down-beat of the next measure.
Some of the advanced CDJs offer the ability to manually mark and play loops within a CD.
Creating loops within the audio track is not usually done during a live DJ performance because of the complexity of the process and lack of time when playing live. This is because in order to loop a certain part of the track, the loop must be perfectly made so that the exact number of measures is marked.
A DJ playing audio tracks can use the above mechanisms to play tracks and mix tracks in a variety of ways, but between playing analog records and using an advanced digital CDJ there is not much difference. In both media each track is played continuously from one point in the track to another. Moreover, in both machines, beat mixing must eventually be done manually. With analog records, the fact that a track is played from one point to another is not surprising. The fact is that the record media is not random access, meaning that it isn't possible to mark and reach any precise desired point in a track on a record at any given time while playing it.
The digital CD on the other hand is indeed random access but the CDJ playing machines attempt to try and imitate the record player rather than use the digital qualities of the digital media, by offering a rotating plate to imitate the touch of a record.
The DJ is therefore limited to the tracks that the DJ has, either original tracks or tracks prepared previously (this is a similar case to marking a loop with an advanced CDJ). The DJ's creativity during a live performance is based on the tracks played and the mixes made (or scratching techniques). The DJ cannot pick different parts of different tracks, loop them or assemble them in a live performance in order to better fit the music to the mood, fit the music to other tracks as well as to be more creative.
Although these described music playing methods let the DJ manipulate the music played to the audience, nevertheless all audio playing mechanisms suffer from a number of disadvantages:
The DJ cannot manipulate the contents of a currently played audio track at a moment's decision in a live situation, without planning and providing for that content manipulation in advance. The DJ can fade out the track and mix it with another but the DJ cannot skip a part of the track, return to a previous part of the track or loop a part of the track on the fly, without planning that manipulation it in advance.
During a mix of audio tracks, the DJ has to constantly monitor the playing tracks and apply corrections to the mix manually.
Before starting a mix, the DJ has to start the new audio track at the precise downbeat that fits with the starting beat of the currently played track. The DJ then needs to correct an inaccurate entry.